This invention relates to the field of optical communications and, more specifically, to Raman-pumped WDM systems.
The demand for communication systems with higher capacities has pushed the common design approaches of wavelength-division-multiplexed (WDM) systems to their limits. A typical configuration of a point-to-point WDM system includes a number of optical transmitters, an optical multiplexer, spans of transmission fiber, optical amplifiers (usually erbium-doped fiber amplifiers, EDFAs), dispersion compensating devices, an optical demultiplexer and a number of optical receivers. Unfortunately, the usable gain bandwidth for the optical amplifiers currently used, for example the EDFAs, is limited and not very broad, and the distortion of the signal does not allow for transmission over very long optical transmission links. This has led to the investigation of alternate methods for amplification with greater broadband capabilities that allow for longer spacing in-between amplification and longer transmission distances.
The use of Raman amplification has been proposed and demonstrated for compensating losses in all-optical transmission systems. Raman amplification is achieved by launching high-power pump waves into a silica fiber at a wavelength lower than the signal to be amplified. Amplification occurs when the pump wavelength gives up its energy to create new photons at the signal wavelength. Since there is a wide range of vibrational states above the ground state, a broad range of transitions may provide gain, of which, typically, 48 nm is usable gain. Raman gain increases almost linearly within the wavelength offset between the pump wavelength and the signal wavelength, peaking at a distance of typically 100 nm and then dropping off rapidly with increased offset. Ultra-broad Raman gain bandwidth can be achieved by combining the Raman amplification effect of multiple pump waves selected carefully for the wavelength domain. See, for example, H. Kidorf, K. Rottwitt, M. Nissov, M. Ma, and E. Rabarijaona, xe2x80x9cPump interactions in a 100-nm bandwidth Raman amplifier,xe2x80x9d IEEE Photonics Tech. Lett. 11, 530, 1999. Additionally, the positions of the gain bandwidth within the wavelength domain of each pump can be adjusted by tuning the pump wavelength. Compared to commonly used erbium-doped fiber amplifiers (EDFAs), Raman amplifiers exhibit several fundamental advantages such as low noise, fixed gain profiles which are independent of signal and pump levels; they are also operable in a plurality of signal bands since Raman gain peak changes with pump wavelength. Despite all of its advantages, there are some degradation effects related to Raman-pumped WDM systems. For example, in addition to the desired pump-to-signal power transfer, there also exist pump-to-pump and signal-to-signal power transfers. The latter two power transfers introduce gain tilting in such a way that signals at longer wavelengths experience stronger gain than those at shorter wavelengths. This effect leads to non-uniform gain and thus the non-uniform nonlinear penalty and noise level across the signal wavelengths. Additionally, power fluctuations in time within the Raman pump wave, which is so often the case, may lead to amplified fluctuations or jitter, which also degrades system performance.
The invention comprises a method and apparatus for achieving flat, broadband Raman gain within a Raman-pumped WDM system by adapting at least one Raman pump in response to differences between a desired gain profile and a determined gain profile.
In another embodiment of the invention, a method for achieving flat broadband Raman gain includes the steps of determining a signal gain profile of a Raman-pumped WDM system and adapting the pump wavelength and/or powers of the Raman-pumped WDM system to modify the signal gain or power profile in a manner approaching a flat or any pre-determined broadband Raman gain or power profile.